When forming a well, such as an oil or natural gas well, a hole is drilled in the earth using an earth-penetrating drill bit situated at an end of one or more drilling tools which, in turn, are disposed at the end of numerous sections of pipe (i.e. a drilling assembly). The drilling assembly is then rotated to affect the drilling process. Typically, a fluid mixture known as mud is circulated into the well during drilling. The mud then flows around the drilling tools and out of the hole along with drilling debris. After the hole is formed a metal casing such as pipe is situated in the hole. Thereafter, cement is provided between the well casing and the hole wall in order to fill space between the outside of the well casing and the hole wall. Once the cement hardens, the well casing is bonded to the hole wall.
It is important for the quality and integrity of a well that no gaps, air pockets or the like exist between the well casing and the hole wall. In order to determine whether gaps, air pockets and/or the like exist between the outer well casing and the hole wall, cement bond evaluation (CBE) is performed. CBE is performed on new wells as well as existing wells by introducing a tool having a CBE component into the well which obtains data that is then analyzed. The CBE component is typically an acoustic (e.g. ultrasonic) transducer configured to operate in a pulse-echo mode. Such is depicted in FIG. 1 wherein a down-hole tool (tool) 10 having an ultrasonic transducer assembly 13 situated in a side thereof is shown sending an ultrasonic signal (represented by the parallel lines) through the mud 11 toward the well casing 14. The ultrasonic transducer assembly 13 includes a piezo-ceramic element situated between an ultrasonic signal dampening backing and a tuned PEEK front face. Behind the well casing 14 is cement 15 and the formation 12 in which the hole was formed.
The ultrasonic transducer 13 generates an ultrasonic pulse that is directed at the well casing 14. The transmitted ultrasonic pulse is reflected off the well casing 14 and returned to the transducer 13. Additionally, a portion of the transmitted ultrasonic pulse travels through the well casing 14 and cement 15, and is reflected off of the formation 12 back to the transducer 13. The ultrasonic reflections cause further ultrasonic reflections that travel back to the transducer. Eventually, the ultrasonic reflections subside. However, by analyzing the reflected ultrasonic signals (i.e. the response signals of the casing and cement), it can be determined whether gaps, air pockets and/or the like, exist in the cement 15 (i.e. between the well casing and the hole wall), as well as the estimated size of the gap, air pocket and/or the like.
Most down-hole pulse-echo ultrasonic transducers operate in the range of 400 kHz to 1 MHz. Selecting an operating frequency for a pulse-echo ultrasonic transducer is a trade-off between transducer size, focal spot size, resolution, ringdown, attenuation and frequency-specific phenomena. The thickness of the well casing is also sensitive to frequency. Transducer size, focal spot size, resolution, and ringdown generally favor higher frequency ultrasonic transducers, while attenuation generally favors lower frequency. There are also more of the specific phenomena at lower frequencies, particularly with respect to well casing (pipe) used for deep wells having a one inch (1″) or greater wall thickness.
Referring to FIGS. 2A-F, various graphed results are shown for tests conducted by the present inventors regarding the effect of ultrasonic frequency on response. Particularly, three ultrasonic transducers of low frequency (115 kHz, 95 kHz, 75 kHz) were used with respect to four (4) well test structures having known gaps in the cement between the well casing and the well formation (i.e. one well test structure having an 0.30 inch gap, one well test structure having an 0.45 inch gap, one well test structure having an 0.71 inch gap, and one well test structure having a 2.33 inch gap—using a section of 6 inch OD steel pipe with a 1 inch thick wall). As seen in graphs 16e of FIGS. 2E and 16f of FIG. 2F, the 115 kHz ultrasonic transducer does not produce responses which allow discrimination between the various gaps. In graphs 16c of FIGS. 2C and 16d of FIG. 2D, the 95 kHz ultrasonic transducer produces responses which allow discrimination of only the 0.71 inch gap from the other gaps. In graphs 16a of FIGS. 2A and 16b of FIG. 2B, however, the 75 kHz ultrasonic transducer produces responses which allow discrimination of all the gaps. Therefore, for down-hole CBE applications where the well casing is relatively thick (i.e. 1″ or greater), lower frequency ultrasonic transducers would be preferable.
A problem with low frequency transducers for CBE applications however is their size since down-hole tools only have so much room for the ultrasonic transducer assembly. Referring to FIG. 3, the problem of size of the ultrasonic transducer assembly is depicted. Particularly, the illustration 17 depicts a 400 kHz ultrasonic transducer assembly 18 and a 100 kHz ultrasonic transducer assembly 19 within the limited space within a down-hole tool. The 400 kHz ultrasonic transducer 18 includes a piezo-ceramic element 20 affixed to an ultrasonic dampening backing with a PEEK face 22 that produces a 400 kHz ultrasonic signal 23.
The 100 kHz ultrasonic transducer 19 includes a piezo-ceramic element 24 affixed to an ultrasonic dampening backing 25 with a PEEK face 26 that produces a 100 kHz ultrasonic signal 27. Every change associated with dropping the operating frequency of the ultrasonic transducer makes achieving bandwidth and ringdown goals more difficult. Particularly, the ratio of ultrasonic dampening backing to the piezo-ceramic element drops, the attenuation in the backing drops by sixteen times (16×), the piezo-ceramic volume versus backing surface area drops by four times (4×), the number of cycles in which to dissipate energy drops by four times (4×) (wherein Q=E/ΔE per cycle), and the thickness versus diameter of the piezo-ceramic approaches 3:1 or worse. As denoted by the double-headed arrow within the piezo-ceramic element 24, the 100 kHz piezo-ceramic element 24 has a significant unwanted radial component.
FIG. 4 illustrates ultrasonic energy flow from the 400 kHz ultrasonic transducer assembly 18 and the 100 kH-z ultrasonic transducer assembly 19. With respect to the 400 kHz ultrasonic transducer assembly 18 (representing typical ultrasonic transducers), ultrasonic energy emitted by the piezo-ceramic element 20 is dissipated in the backing 21 as represented by the large arrow 30. Particularly, nearly 50% of the ultrasonic energy projected into the backing 21 is dissipated, while a 5%-10% heat loss occurs in the piezo-ceramic 20. Moreover, as represented by the two small arrows 31, there is minimal coherent echoing from the backing 21. As represented by the large arrow 32 labeled “Transmitted”, the remainder of the ultrasonic energy is emitted by the piezo-ceramic element 20. With respect to the 100 kHz ultrasonic transducer assembly 19 (representing low frequency ultrasonic transducers), some of the ultrasonic energy emitted by the piezo-ceramic element 26 is dissipated in the backing 21 (as represented by the large arrow 34), but not as much as the 400 kHz ultrasonic transducer assembly 18 since the backing 25 of the 100 kHz ultrasonic transducer assembly 19 is smaller. Moreover, as represented by the two small arrows 35, there is more echo resulting from the backing 25. As represented by the large arrow 36 labeled “Transmitted”, the remainder of the ultrasonic energy is emitted by the piezo-ceramic element 26.
Therefore, while it would be advantageous to have a low frequency (i.e. ˜100 kHz) ultrasonic transducer for use in CBE and other applications, the aforementioned problems with respect to low frequency (i.e. 100 kHz) ultrasonic transducers makes current designs unsuitable and/or unusable in down-hole CBE applications. As the ability of the backing to absorb energy from the transducer decreases (due to limited space in a downhole tool for a transducer assembly), loss in the transducer must be increased to meet bandwidth goals.